It kind of depends on how your going to be treating your equipment. If you know right now that you won't be maintaining your gear, throwing it in the back of your car after a game and leaving it in your bag until the next game without any trace of cleaning or lubrication, then the tippy is the way (I'd echo ruthless' sentiments here, the tippman simply will not break down). BUT, if you take care of it, clean and lube it regularly, and throw in some new seals every now and then then the E99 will treat you better. The recent advances in entry level electros have been simply amazing, they are so damn cheap and really are decent markers; while the tippman may be rugged, but it really is a solid chunk of aluminum with a design that hasn't changed much at all in 5 years (they're overpiced if you ask me).
And I's have to disagree with ruthless about the dependability of electros, IF you take care of them and keep a fresh battery in, they will treat you fine (One time I accidentally took my Black Dragun swimming in a really deep puddle and came out firing, a little wipe down of the electronics and any rustable parts and it was fine). The fact is, electro markers are fast becoming the way to go now, while pneumatic designs haven't really changed much in 5-6 years.